The objective of the proposed research is to demonstrate the feasibility of an ultrawideband space-time microwave imaging (STMI) method for early-stage breast cancer detection. The outcome of the proposed work will be 1) the development of robust STM about signal processing algorithms for detection and imaging of malignant lesions, 2) STMI performance evaluation using realistic numerical breast phantoms, 3) realistic physical breast phantoms and a first generation STMI hardware prototype, and 4) experimental STMI performance evaluation using physical phantoms and ex vivo human breast tissue imaging scenarios. Two classes of STMI algorithms will be considered: beamforming and adaptive detection. The beamformer combines filtered versions of the backscattered signals at an antenna array to pass signals scattered from a given location with unit gain while attenuating backscattered signals from other directions. The beamformer focus location is scanned to generate a map of scattered signal energy as a function of position. Adaptive detection methods form a test statistic based on the expected space-time pattern that would result if a lesion were actually located at the candidate location. Both techniques will incorporate frequency-dependent propagation and scattering effects and optimally discriminate against artifacts and noise. Anatomically realistic numerical breast phantoms will be derived from routine high resolution 3-D MRI breast scans. The prototype instrument will be constructed from a commercial vector network analyzer and an ultrawideband antenna that is repositioned to create a synthetic array. The physical breast phantom will consist of heterogeneous solid tissue simulants molded into the shape of a naturally flattened breast. Ex vivo imaging experiments will be conducted on six different patients' tissue samples obtained from reduction surgeries. The proposed ultrawideband STMI approach offers the potential of a pain-free non-ionizing breast cancer screening technology capable of reducing the rate of false negatives and false positives associated with conventional X-ray mammography, especially for challenging cases involving radiographically dense breasts or tumors near the chest wall or near the axilla.